1. Field of the Invention
The present invention relates generally to aircraft structural members, and more particularly to sheet and plate suitable for wide body commercial aircraft fuselages as well as associated methods.
2. Description of Related Art
The fuselage of wide body commercial aircraft is typically composed of a skin made of AlCuMg type alloy metal sheet or plate, and longitudinal stiffeners (stringers) and circumferential frames. A frequently used alloy is type 2024, which has the following chemical composition (% by weight) according to the Aluminum Association designation or to standard EN 573-3:
Si<0.5, Fe<0.5, Cu 3.8-4.9, Mg 1.2-1.8, Mn 0.3-0.9, Cr<0.10, Zn<0.25, Ti<0.15.
Variants of this alloy are also used. These structural members are expected to provide a compromise between several properties such as mechanical strength (i.e. static mechanical characteristics), damage tolerance (fracture toughness and cracking rate in fatigue), fatigue resistance (particularly oligocyclic), resistance to different forms of corrosion, and formability. Resistance to creep can be critical in some cases, particularly for supersonic aircraft.
Various alternative solutions have been proposed in order to improve the compromise between the various required properties, and particularly mechanical strength and toughness. Boeing has developed the 2034 alloy with composition:
Si<0.10, Fe<0.12, Cu: 4.2-4.8, Mg 1.3-1.9, Mn 0.8-1.3, Cr<0.05, Zn<0.20, Ti<0.15, Zr 0.08-0.15.
This alloy is disclosed in patent EP 0 031 605 (U.S. Pat. No. 4,336,075). It has a better specific yield stress than 2024 in the T351 state, due to the increased contents of manganese and the addition of another anti-recrystallising agent (Zr), and has improved toughness and resistance to fatigue.
U.S. Pat. No. 5,652,063 (Alcoa) relates to an aircraft structural member made from an alloy with composition (% by weight):                Cu: 4.85-5.3, Mg: 0.51-1.0, Mn: 0.4-0.8, Ag: 0.2-0.8, Si<0.1, Fe<0.1, Zr <0.25, where Cu/Mg is between 5 and 9.        
Sheet metal made from this alloy in the T8 state has a yield stress >77 ksi (531 MPa). The alloy is intended particularly for supersonic aircraft.
EP Patent 0 473 122 (U.S. Pat. No. 5,213,639) by Alcoa discloses an alloy recorded by the Aluminum Association as 2524, with composition Si<0.10, Fe<0.12, Cu 3.8-4.5, Mg 1.2-1.8, Mn 0.3-0.9, that may possibly contain another anti-recrystallising agent (Zr, V, Hf, Cr, Ag or Sc). This alloy is intended particularly for thin sheets for a fuselage and has better toughness and resistance to crack propagation than 2024.
EP Patent Application 0 731 185 assigned to Pechiney Rhenalu relates to an alloy subsequently recorded under No. 2024A, with composition Si<0.25, Fe<0.25, Cu 3.5-5, Mg 1-2, Mn<0.55 with the relation 0<(Mn-2Fe)<0.2. Thick plates made of this alloy have improved toughness and low residual stresses, without any loss of other properties.
U.S. Pat. No. 5,593,516 (Reynolds) relates to an alloy for aeronautical applications containing 2.5 to 5.5% Cu and 0.1 to 2.3% Mg, in which the contents of Cu and Mg are kept below their solubility limit in aluminium, and are related by the following equations:Cumax=5.59−0.91 Mg and Cumin=4.59−0.91 Mg.
The alloy may also contain Zr<0.20%, V<0.20%, Mn<0.80%, Ti<0.05%, Fe<0.15%, Si<0.10%.
U.S. Pat. Nos. 5,376,192 and 5,512,112, relate to alloys of this type containing 0.1 to 1% silver. Note that the use of silver in this type of alloy increases the production cost and introduces difficulties in recycling of fabrication waste.
EP Patent Application 1 170 394 A2 (Alcoa) describes four types of AlCu alloys with the following composition, respectively:                Cu 4.08, Mn 0.29, Mg 1.36, Zr 0.12, Fe 0.02, Si 0.01;        Cu 4.33, Mn 0.30, Mg 1.38, Zr 0.10, Fe 0.01, Si 0.00;        Cu 4.09, Mn 0.58, Mg 1.35, Zr 0.11, Fe 0.02, Si 0.01; and        Cu 4.22, Mn 0.66, Mg 1.32, Zr 0.10, Fe 0.01, Si 0.01.        
The '394 patent describes how to transform these products into sheet metal with an elongated grain structure, in which the grains have a length to thickness ratio of more than 4. If a certain, specific microstructure and a clearly defined texture are obtained, this product has good mechanical strength properties and damage tolerance. One of the disadvantages of these alloys is that they are based on high purity aluminium (very low silicon and iron content), which is expensive. Another Alcoa patent, U.S. Pat. No. 5,630,889, discloses sheet metal in the T6 or T8 state made of an AlCuMg alloy containing:
Cu 4.66, Mg 0.81, Mn 0.62, Fe 0.06, Si 0.04, Zn 0.36%.
The addition of silver is said to improve the properties of this alloy. However, silver is an expensive element and it limits the recycling of products obtained in this way and production waste from these products, which even further contributes to increasing the cost price of the products.